coolant noise/leak?

Lately I have been hearing a water noise in the dash area when turning or moving. Is my heater core leaking and filling up my heater box? I have had to add antifreeze to the overfill jug, but I don't see any external leaks. Sometimes I do get a whiff of coolant when I turn the heater on. If so, is it a big job to repair? I am hoping it is something else but, I don't have any other ideas. Is there something I should look for? Should I pressure test the cooling system?

To my knowledge the dash has to be removed for the core to be replaced. you might check under the hood on the pass. side of the firewall just to make sure your drain tube hasn't been turned upward. There is a fairly good video on you-tube on core replacement.